1. Field of the Invention
The present invention relates to the protection of refractory composite materials against corrosion, and more particularly against the action of oxygen or possibly water at high temperature.
The composite materials to which the invention applies are materials comprising refractory fibrous reinforcement densified by a matrix that is likewise refractory.
2. Prior Art
The material constituting the fibers of the fibrous reinforcement is generally carbon, or a ceramic such as silicon carbide, for example. The refractory matrix is constituted by carbon, or at least in part by a ceramic such as silicon carbide or a refractory oxide, for example. An interphase, in particular boron nitride or pyrolytic carbon, may be formed on the reinforcing fibers to ensure adequate bonding between the fibrous reinforcement and the matrix.
The fibrous reinforcement is densified with the matrix using a liquid technique or a gaseous technique, the densification serving to fill in, at least partially, the accessible pores of the fibrous reinforcement, throughout the volume of the latter.
With a liquid technique, the fibrous reinforcement is impregnated by a matrix precursor, e.g. a resin. After impregnation, heat treatment is applied during which the material constituting the matrix is obtained by a transformation of the precursor. Several successive impregnation and heat treatment cycles may be performed.
With the gaseous technique, the fibrous reinforcement is placed in an enclosure into which is admitted a gas that decomposes or reacts under particular temperature and pressure conditions to form a deposit on the reinforcing fibers, and throughout the volume thereof. Methods of chemical vapor infiltration of refractory materials are well known, notably in the case of ceramic materials, the infiltration optionally being conducted after forming a fiber-matrix bonding interphase. Reference may be made to the following documents: FR-A-2 401 888, EP-A-0 085 601, and EP-A-0 172 082.
Refractory composite materials are remarkable because of their thermostructural properties, i.e. mechanical properties which make them suitable for constituting structural components, and because of their ability to conserve these mechanical properties up to high temperatures. These materials are thus used in aeronautical and space applications, in particular.
However, if refractory composite materials are placed in an aggressive atmosphere during use, they are prone to damage by corrosion attacking the fibers of the reinforcement, the fiber-matrix interphase, or the matrix.
One type of corrosion that produces particularly severe problems in practice is the action of oxygen or the combined action of oxygen and water that occurs when the fibrous reinforcement or the matrix of such materials contain oxidizable substances (such as carbon, boron nitride, . . . ) and that are raised to high temperature in the presence of air, combustion gases, humidity, rain, . . . . This happens, for example, to the materials constituting the combustion chamber of a turbojet, or to the outside portions of space vehicles on re-entry to the atmosphere.
The action of corrosive agents is enhanced by the practically inevitable cracking of refractory composite materials. When such materials are used, cracks (generally microcracks) appear because of mechanical stresses applied to the materials or because of a difference in thermal expansion coefficients between the fibrous reinforcement and the matrix.
As shown very schematically in accompanying FIG. 1, each crack allows corrosive agents to access not only the material of the matrix M, but also fibers F, which may optionally be sheathed by an interphase I. Because of the almost inevitable residual porosity of the material (densification of the fibrous reinforcement is rarely complete), this phenomenon occurs not only at the surface, but also in the core of the material, with the corrosive agents being conveyed into the pores.
For high temperature applications in contact with air, and possibly in the presence of humidity, it is necessary to protect the refractory composite materials.
The state of the art concerning the protection of refractory composite materials, and in particular the protection of composite materials containing carbon against oxidation, is most abundant. The techniques used often rely on forming a protection that has healing properties for the purpose of plugging, filling, or sealing cracks that appear in the material. While the material is in use, variations in thermal and mechanical stresses give rise to variations in the shapes of the cracks, and in particular to a widening or a narrowing of the gaps between their lips. It is therefore necessary for the healing protective material to be capable of following such movements without itself cracking. For this reason, the protective material is usually a substance that forms a glass or that is capable of forming a glass, e.g. after being oxidized, the glass having viscous behavior at the temperature at which the material is used.
As shown in FIGS. 2 and 3, the glass V provides a protective function by forming a barrier preventing access of the corrosive agents into the cracks in the material. In FIG. 2, the glass V is made of substances deposited on the composite material. In FIG. 3, the glass V is formed by a corrosion (oxidation) of substances contained within the matrix material.
Refractory composite materials can be protected against oxidation by depositing a layer made of a silicon compound and/or by depositing a layer made of a boron compound, thereby forming a glass based on boron oxide (B.sub.2 O.sub.3) or based on silica (SiO.sub.2) or on a combination of both. For an illustration of this state of the art, reference may be made to the following documents: U.S. Pat. No. 4,668,579 and EP-0 176 055.
In Document U.S. Pat. No. 4,668,579 (inventors Strangman, et al.), a carbon-carbon (C/C) composite material is protected against oxidation by forming at least one protective layer comprising an inner boron carbide portion and an outer silicon carbide portion. The protective layer is preferably formed before complete densification of the composite material, typically after a step for consolidating the fibrous reinforcement, i.e. after partial densification has been performed to an extent which is just sufficient for bonding the reinforcing fibers together. The thickness of each portion of the protective layer lies in the range 0.5 microns to 5 microns (0.2 to 2 mils).
In Document EP-A-0 176 055 (inventors Holzl, et al.), a carbon body (which may be a C/C composite) is protected against oxidation by an initial chemical etching of the carbon body with a boron oxide to form interstices which extend down to a determined depth and which occupy about one-half of the initial volume of the carbon body down to that depth. The porosity created in this way is filled by inserting silicon or a silicon alloy, which, by reaction, gives rise to a layer made of substantially equal parts of silicon boride and silicon carbide. An additional surface coating, e.g. of silicon carbide, is formed with or without an intermediate layer of boron or of a boron compound. The carbon body treated in this way provides very good resistance to oxidation in air at a temperature of about 1370.degree. C.
Patent documents U.S. Pat. Nos. 4,889,686 and 4,944,904 also concern the manufacture of a composite material protected from the oxidation by the introduction of a molten silicon compound within a porous preform. The preform is made from fibers having a boron nitride coating to prevent direct contact with the molten silicon, and in some cases an additional pyrolytic carbon coating to enhance the wetting by the silicon. The thus-coated fibers are then impregnated by an infiltration promoter in the form of short carbon fibers, or silicon carbide whiskers, and/or carbon powder mixed with a binder, such as resin. A fibrous preform is prepared by setting the impregnated fibers into shape and submitting them to a thermal treatment, after which a liquid infiltration is carried out using a mixture of silicon and boron in a molten state, the quantity of boron in this mixture preferably being in a ratio of 1 to 3% by weight with respect to the silicon. The resulting composite material comprises a matrix with a principal phase constituted by silicon carbide formed in situ, or by silicon carbide containing boron and formed in situ, and an additional phase constituted by a silicon and boron solution.
Among the glasses susceptible of being formed as a result of implementing an anti-oxidation process, those based on boron have limited low pressure performance and are sensitive to the effects of humidity. The same is not true of glasses based on silica, but silica glasses are not effective at medium temperature (because their viscosity is too high).
Glasses based on a mixture of boron and silica, i.e. "borosilicate" glasses do not suffer from those drawbacks, or at least they are greatly attenuated. Juxtaposing layers of a precursor for boron-based glasses and a precursor for silica-based glasses, e.g. an inner layer of boron carbide (B.sub.4 C) and an outer layer of silicon carbide (SiC) as in the state of the art mentioned above, would yield a boro silicate glass.
As shown in highly diagrammatic form in FIG. 4, when two layers of B.sub.4 C and SiC are superposed and exposed to oxidizing conditions, corresponding oxides (B.sub.2 O.sub.3 and SiO.sub.2, respectively) are formed on the walls of a crack, at the level of the B.sub.4 C, and SiC layers. These oxides are formed adjacent to each other along the crack. There is no immediate formation of borosilicate type glass. Thus, at an initial oxidation stage, the above-mentioned defects of boron-based glasses and of silica-based glasses are to be found juxtaposed. In addition, the B.sub.4 C and the SiC layers do not oxidize at the same rate and they do not have the same physico-chemical properties; in particular they do not have the same thermal expansion coefficients.
Thus, in a high temperature oxidizing atmosphere, it is observed that the oxide B.sub.2 O.sub.3 advances at the interface between the B.sub.4 C and the SiC layers as shown in FIG. 5. If the composite material is then placed in the presence of humidity, the oxide B.sub.2 O.sub.3 hydrates into B.sub.2 O.sub.3, n-H.sub.2 O which is greater in volume than the oxide that gave rise to it. This tends to split the interface between B.sub.4 C and SiC perpendicularly to the layers, thereby separating the layers (FIG. 6). At worst, the outer layer of SiC flakes off, in any case, there will at least be cracking at the interface between B.sub.4 C and SiC, and a repetition of that phenomenon. These phenomena encountered with protections that associate layers of B.sub.4 C and SiC are described, in particular, in an article by C.W. Ohlhorst, et al., entitled "Performance evaluations of oxidation-resistant carbon-carbon composites" (Fifth National Aerospace Plane Symposium, Oct. 18-21, 1988, Paper No. 69).
Thus, an object of the present invention is to provide a process for obtaining refractory composite material protected against corrosion over a wide temperature range, and up to at least 1700.degree. C., by formation of at least one healing layer of borosilicate glass when the cracked material is placed under oxidizing conditions at high temperature.